Let's Agree to Head Toward a Dual-mode Solution

Maybe I'm missing something here, but there
seems to be more agreement going on in this debate than
disagreement.

It appears to me that a major point of
Vukan Vuchic is that low density suburbs tend to have
both O/D patterns which are temporally and spatially
dispersed, and existing guideway infrastructure (e.g. roads)
which are not used to capacity. This is the reason that
individual traveler operated vehicles have evolved to provide
wide-area transportation service to these regions, and the
influences behind this trend, will probably continue. I don't
think anyone qualitatively believes that a new replacement
system could be deployed to serve all the needs for these low
density areas; its just a quantitative question of the
optimal density vs. cost which would substantiate a new
system deployment.

I think everyone agrees that CBDs and capacity
constrained corridors represent the highest priority for
replacement of traveler-operated vehicle-generated
congestion. Everyone agrees that a dedicated guideway which
minimizes conflicts with the existing system can provide
higher throughput, convenience, and time savings, but will
require substantial initial deployment costs. Everyone agrees
that an automated system decreases labor costs and increases
traveler convenience, but will require a separate guideway.
So if we are going to build a new system with a new guideway,
it might as well make maximum use of automation.

The primary focus of controversy appears to be in regard
to the size of the vehicles for the new deployment, the
degree to which they should be automated and how travelers
will access the new system.
Vukan Vuchic appears concerned that a lot of automated
small vehicles running around will be more difficult
logistically to handle than a smaller number of large
vehicles, will require small (and therefore potentially
unsafe) headways to get good throughput, and will have a
500(+) Kg weight-to-traveler ratio which will increase
emissions and/or energy consumption.

Edward Anderson sites various models which indicate
that the logistics of even CBD surge loading can be handled
with adequate system design for a small vehicle automated
vehicle system. He indicates that such a system can
inherently service a wider spatial and temporal range of
O/Ds, and therefore has great utility and versatility.

Martin Bernard pointed out that a problem with the
deployment of any new transportation system is that travelers
will tend to use the existing (SOV based) system to access
it, and therefore either large parking lots have to be
constructed, or publicly available station cars may be used.
He mentions that many modal options may be applicable (e.g. a
true multi-modal system), but seems to indicate that a
dual-mode PRT type system might be the most appropriate for
corridors of more then a few miles that don't have existing
rail tracks.

Dennis Manning points out that a large fleet of PRTs
would have the convenience of a fleet of privately
chauffeured taxis without the labor costs. Dennis also states
that such a system might do well to borrow some operational
concepts from AHS.

Palle Jensen expands upon the benefits of a dual-mode
PRT in that it can do everything that a captive PRT can do,
but also increases accessibility to those low density O/Ds
that Prof. Vuchic is so concerned about. As we all know,
Palle has some rather detailed suggestions on how this might
come about (see the
RUF concept description).

So it would appear that a dual-mode PRT with
small-to-medium size vehicles may provide an ideal mix of
speed, throughput, convenience, and accessibility. The
vehicles would only have to accelerate once between origin
and destination and could wind draft at close headways, which
would reduce both emissions and energy consumption. However,
I'm sure Prof.. Vuchic (as well as everyone else) would be
concerned with the safety and technical feasibility of close
headway spacing for a stream of vehicles capable of both
automated and traveler-based control. From the recent
PRT conference , we know that
Michel Parent,
Mark Buehrer,
Jay Andress, and (of course) Palle Jensen all support
some aspect of this dual-mode idea. We also know from
Jerry Schneider's paper (and others) that cost is an
extremely critical factor for real world implementations.

So how could a dual-mode PRT be designed with 1) low
headways, 2) high safety, 3) low guideway costs, and 4) low
vehicle costs? These issues are worth examining.

In regard to motorized vehicles, you can't beat the
purchase price of autos. A number of manufacturers are
already tooled up and competing to produce them at $10K -
$20K a pop, and it is unreasonable to expect a comparably
sized vehicle to be produced for less then this. Many, if not
most, new vehicles come off the line with computer controlled
throttles, computer controlled (ABS) brakes, and power
steering. The marginal costs of making these things
completely computer-controlled is not much - certainly much
less then designing a whole custom new vehicle. Also, a fully
loaded passenger van weighs in at considerably under 500 kg
per seat, which I'm sure would warm the heart of Prof..
Vuchic.

Most
PRT designs assume an elevated guideway in order to
assure the automated vehicles are not impeded by
unanticipated outside influences. However, there are already
existing facilities closed off from the outside world that
have contiguous right-of-way (ROW) directly along the
corridors with the heaviest demand- they're known as
freeways. Being as the economic, political, and institutional
costs of installing a guideway through multiple political
boundaries could be enormous, a narrow freeway medium (for a
narrow PRT guideway) may not be a bad choice. Few things
could provide more incentive to get SOVs into a PRT than to
have the PRT vehicles whiz by them every morning as they are
stuck in traffic.

The guideway structural cross section need only
substantiate the weight of small PRT (and possibly freight)
vehicles, which is a few orders of magnitude less than the
design standards for freeways (which must support overloaded
trucks). This minimizes the need for extensive geotechnical
foundations, substantially reducing the construction time and
cost. Because a PRT is narrow enough that it does not require
widening of over-crossings (as do conventional freeway lane
expansions) and because it would not interface with the
conventional roadway system at capacity constrained
interchanges (which cause huge SOV traffic control and
construction phasing problems), the guideway construction
could be quite fast, simple, and (most importantly) cheap. An
extended elevated guideway could be expanded to city centers,
airports, etc. as necessary; but the majority of the guideway
needs to be inexpensively built (right-of-way, traffic
control, construction, and political costs) to make an
initial PRT implementation FEASIBLE.

But how do you get a conventional auto vehicle design to
run on an automated PRT guideway at low headways with high
safety? Well, some in the
AHS world would say that you run vehicles (modified
for complete automated control) along something analogous to
a conventional freeway lane. Unfortunately, these lanes are
too wide (e.g. suck up too much expensive ROW). Because the
vehicles are not laterally constrained, a vehicle component
failure or unexpected piece of debris could cause the
closely-spaced following vehicles to careen into each other,
crashing or rolling laterally. Also, these vehicles
inherently would have to have some type of command and
control (C&C) functions coming via RF through the air
waves, making them susceptible to the safety problems of
hacker terrorists and disgruntled nerds.

These problems are mitigated if we assume the lane is
only slightly wider then the vehicle. Even if the vehicle is
totally out of control, the worst it could do is toboggan to
a stop. In fact, if we are going to have a physical barrier
on both sides of the lane, we might as well use that as a
continuous control surface to guide the vehicle (so that it
normally never physically contacts anything). This close
proximity to the guide rail means that a low power
"leaky cable" continuous antenna can be used
(without a FCC license) to provide virtually unlimited RF
communication bandwidth to the vehicles for physically
hack-proof C&C functions, as well as entertainment
functions. {Travelers are going to want the capability to
inexpensively watch cable TV, surf the net, video conference,
play games, etc. in the otherwise non-productive time as they
are being whisked to/from work}

Because this PRT concept has vehicles that are laterally
constrained, guideway structural support need only be
supplied directly under the wheels, making both ground-based
and elevated guideway construction much simpler and cheaper.
This physical configuration of load bearing and guiderail
cross-section forms the guideway into a natural truss,
providing both vertical and lateral stability for elevated
sections (which is kind of important to us out here on the
earthquake prone Pacific Rim) using the minimal amount of
steel. The low weight design not only maximizes the span
length, but allows pre-fabrication off the construction site,
using "off-the-shelf" cross-sections, thereby
decreasing construction costs.

What about headway, safety, and vehicle breakdowns? Well
the platoon headway spacing should be pretty close to zero
because 1) this would reduce energy consumption, and 2) if in
the unlikely case a vehicle had a catastrophic mechanical
failure where the drive train suddenly locked up, there would
be minimal impact speed between vehicles. After the initial
contact, the vehicles following in the platoon would simply
slow down and push the broken down vehicle to the nearest
egress station. However, this might scrape the paint off the
disabled vehicle, so an additional safety feature would be
helpful (see below).

Conventional rail systems, including
monorail and Palle Jensen's
RUF system, achieve both lateral control and weight
bearing by physical contact with the rail(s). This causes
mechanical wear, noise, and lubrication problems. Also, minor
lateral imperfections in the rail construction can cause the
vehicle to sway as it traverses the track, the momentum of
which then induces greater lateral strain on the rail,
eventually leading to (traveler disturbing) vehicle vibration
and oscillations. This situation is avoided if we use a
rubber tired vehicle with electronic lateral control (via a
cheap IR distance transducer off the side guide rails) and
exactly match the vehicle velocity to the super-elevation.
{Smooth ridability is a desirable feature to travelers.}
However, to assure absolute safety in case of a catastrophic
control system failure, a mechanical fail-safe system would
be useful.

It would be useful to have a small bracket extend from
the vehicle to wrap around the guiderail (sorry, I can't
provide a picture here) upon check-in procedure to the PRT.
Although no part of this bracket would normally contact the
guiderail for normal operation, it would provide an absolute
fail-safe safety feature, as well as providing a convenient
mount for the lateral offset control transducer. A right
bracket would be extended to the right guiderail for vehicles
preparing to access or egress the PRT. For mainline travel,
the right bracket is retracted and the left bracket is used
for primary control, thereby bypassing off-ramps. A
physically contacting version of these brackets could also be
used as an electrical pick-up off the guiderail to charge up
EVs (or Zero Emission Vehicles-ZEVs), for a truly
environmentally benign form of transportation. Indeed, some
type of inter-city guideway electrical pick up may be
necessary to make ZEVs into a viable consumer market and
allow them to achieve the market share directed by some
legislation.

The wrap around brackets could also be used for emergency
breaking, greatly reducing the stopping distance over
conventional tire-only breaking. Also, in the event of a
control system or a mechanical breakdown, these brake pad
surfaces would not abrade the guiderail. These small
extendible vehicle brackets would be the only physical
modification needed to conventional production vehicles
(other than power steering actuator control). I think the
additional feeling of safety provided the travelers (and
liability protection provided the operators) would be well
worth this small marginal cost.

Even if someone shot out all the tires of a moving
platoon of vehicles on a turning elevated guideway during a
freezing rain blizzard when there was a complete power and
computer system failure during an earthquake - no traveler
could be injured. This may be no small selling point.

It is possible to go on and on, further cultivating this
dual-mode concept, but that's not really the point here.
{These ideas have been much further developed then might
be implied here. Left out are a number of important details,
such as an inexpensive and reliable mechanism to exactly
longitudinally sync the vehicles within a platoon, find a
non-communicating lone stalled vehicle, find any other
impediment that might intrude upon the guideway; construction
details; geometric design considerations; etc. These issues
may lie beyond the immediate interest level of the reader.
Further details can be supplied as appropriate.}

The point is that I believe perhaps the most robust new
transportation option is not just a viable combination of
what Prof.. Vuchic, Mr. Bernard, and others have said here,
but should include some of the more pragmatic ideas from
other domains as well. A distillation of the most useful of
these ideas, along with some of the more practical aspects of
AHS thinking could, as Mr. Manning indicates, produce a more
viable option then either the PRT or
AHS communities alone have come up with so far.

Joe Palen is a Professional Engineer who has been
involved in transportation system development for the past 20
years.